Electromagnetic induction has been known for a long time and it has been used in many applications such as generators, electronic motors and transformers. In electromagnetic induction a time-varying magnetic flux induces an electromotive force to a closed conductor loop. Vice versa, a time-varying current creates a varying magnetic flux. In transformers, this phenomenon is utilized to transfer energy wirelessly from one circuit to another via inductively coupled coils. A primary coil transforms an alternating current into a varying magnetic flux, which is arranged to flow through the secondary coil. The varying magnetic flux then induces an alternating voltage over the secondary coil. The proportion of the input and output voltage can be selected by configuring the number of turns in the primary and secondary coils.
Wireless power transfer is another application where electromagnetic induction is used to transfer energy over the air. A wireless power transfer system may for example comprise a pair of coils coupled to each other for transferring energy by means of electromagnetic induction. A wireless power transfer system may comprise a transmitter device with a primary coil, and a receiver device with a secondary coil. For example, the transmitter may be a wireless charger device with the primary coil inductively coupled to a secondary coil of the receiver device. The current in the charger device is transferred to the receiver device through the electromagnetically coupled primary and secondary coils. The induced current may be further processed in the receiver device, for example to charge a battery. The secondary coil may be removably coupled to the primary coil, for example where the secondary coil is in a portable device.